The present invention generally relates to wireless communication networks, and particularly relates to the use of channel coding resources on shared packet data channels.
Wireless communication networks based on Code Division Multiple Access (CDMA) generally transmit different streams of information on different code channels using orthogonal or quasi-orthogonal codes. Walsh codes of varying length stand as one example of such codes, and they find widespread use in wireless networks based on cdma2000 and Wideband CDMA (W-CDMA) standards, for example.
A traditional approach to using Walsh codes involves the assignment of specific Walsh codes of desired lengths to each user. Accordingly, one or more of the available Walsh codes at a given network transmitter are used to create the dedicated code channel(s) assigned to each user, and additional codes are dedicated as needed for the various broadcast, control, and overhead signaling channels. Control logic in the network handles allocating and deallocating individual codes from the available set of codes dynamically in response to users being added or dropped from the network on an ongoing basis.
Newer network standards depart from the traditional approach of dedicating Walsh code resources to individual users, at least on the forward data link. For example, IS-2000 Revisions C and D represent attempts at satisfying the ever-present push for higher speed mobile data services. These revisions, often referred to as “1xEV-DV” (shorthand for 1× radio transmission technology Evolution for high-speed integrated Data and Voice), introduce a high-speed shared forward packet data channel, referred to as the “Forward Packet Data Channel” or F-PDCH, that uses multiple Walsh codes to provide high-speed packet data services to the users sharing that channel. The High Speed Downlink Packet Access (HSDPA) channel of W-CDMA makes similar use of multiple codes to increase the available data rates on a shared packet data channel.
In 1xEV-DV, the F-PDCH is a rate-controlled channel rather than a power-controlled channel. It comprises an ongoing series of time slots—e.g., 1.25 ms/slot—with each slot of the F-PDCH generally comprising a multi-coded data transmission targeted to a given one of the mobile stations sharing the F-PDCH. Some type of “scheduler” generally manages time-sharing of the channel, so that each user sharing the channel can be served within the constraints of that user's radio conditions. In particular, the data rate used to serve each user at any given instant depends on the radio conditions dynamically reported by that user, the transmit power currently available for the F-PDCH, and the number of Walsh codes currently available for the F-PDCH. Scheduling and other transmission information associated with the F-PDCH is sent to the mobile stations sharing that channel via a Forward Packet Data Control Channel (F-PDCCH).
For a given transmit power availability, the maximum data rate achievable on the F-PDCH at any given instant depends on the number of Walsh codes available to it. Coding on the F-PDCH uses multiples, not necessarily contiguous, of length-32 Walsh codes from the defined Walsh (code) space. This approach is called multi-code CDMA (MCCDMA), and the more codes available for multi-coding the F-PDCH transmissions, the higher the achievable data rate.
In contrast, in previous revisions of the IS-2000 standards, Walsh code resources were solely dedicated to individual users, and higher data rates for individual users were achieved by increasing the length of the Walsh codes dedicated to them. Hence, to make the multi-coding methodology used for the F-PDCH backwards compatible, the F-PDCH is configured to use the “leftover” Walsh codes not dedicated to other channels. With this approach, Walsh codes are assigned as needed to control and signaling channels, and to traditional voice and data users, with the remaining Walsh codes available for use on the F-PDCH. More particularly, the F-PDCH can use up to 28 length-32 Walsh codes, with the rest of the Walsh codes used for control and signaling purposes, and for conventional voice/data channels.
Mobile stations receiving data on the F-PDCH have to know exactly which Walsh codes are being used in multi-coding the transmissions. Theoretically, the Walsh codes used for the F-PDCH can change every 1.25 ms—the “slot” timing of the F-PDCH—as the conventional voice/data channels occupy or abandon Walsh codes with the arrival or departure of conventional voice/data users. Randomness in that arrival/departure process creates fragmentation in the Walsh space, resulting non-contiguous leftover codes in the Walsh space. Hence, the typical network Base Station (BS) transmits a broadcast message on the F-PDCCH to apprise mobile stations of the ever-changing allocation of leftover Walsh codes being used for the F-PDCH.
Thus, running the F-PDCH at the highest data rates permitted by the instantaneously prevailing conditions—transmit power availability, user radio conditions, etc.—depends on adding length-32 Walsh codes to the multi-coding set used for the F-PDCH, as soon they become available. However, adding a Walsh code the multi-coding set generally requires notifying the mobile stations of the change.
The 1xEV-DV standard defines a Walsh Mask Broadcast (WMB) message that is transmitted on the F-PDCCH, and which comprises a “bit map” marking pairs of length-32 Walsh codes available for multi-coding on the F-PDCCH. The WMB message comprises 21 bits and the time consumed on the F-PDCCH by its transmission displaces control information needed for controlling the F-PDCH. Transmission of the WMB message thus imposes a throughput penalty on the F-PDCH.
At the same time, however, transmission of the WMB message offers a throughput benefit in that it allows the mobile stations to be informed of one or more additional Walsh codes newly available for multi-coding on the F-PDCH. One challenge, then, lies in determining at any given time whether the benefit of transmitting an updated WMB message outweighs the penalty that will be incurred by its transmission.